I'll detail the code here, so you can build your own and adapt it to your needs.

First, we need to import GTK+ and pynotify, and initialize it.

import gtk
import pynotify
pynotify.init(sys.argv[0])

Then, we need to import gdata Calendar API and connect to the calendar. I'll use the simple email/password way to login, which is clearly not the best, but it's also the simplest. Feel free to use OAuth 2.0. :-)

Now we're ready to request stuff and notify! First, request the events from the default calendar.

feed = calendar_service.GetCalendarEventFeed()

Now we can iterate over feed and do various checks.

for event in feed.entry:
# If the event status is not confirmed, go to the next event.
if event.event_status.value != "CONFIRMED":
continue
# Now iterate over all the event dates (usually it has one)
for when in event.when:
# Parse start and end time
try:
start_time = datetime.datetime.strptime(when.start_time.split(".")[0], "%Y-%m-%dT%H:%M:%S")
end_time = datetime.datetime.strptime(when.end_time.split(".")[0], "%Y-%m-%dT%H:%M:%S")
except ValueError:
# ValueError happens on parsing error. Parsing errors
# usually happen for "all day" events since they have
# not time, but we do not care about this events.
continue
now = datetime.datetime.now()
# Check that the event hasn't already ended
if end_time > now:
# Check each alert
for reminder in when.reminder:
# We handle only reminders with method "alert"
# and whose start time minus the reminder delay has passed
if reminder.method == "alert" \
and start_time - datetime.timedelta(0, 60 * int(reminder.minutes)) < now:
# Build the notification
notification = pynotify.Notification(summary=event.title.text,
message=event.content.text)
# Set an icon from the GTK+ stock icons
notification.set_icon_from_pixbuf(gtk.Label().render_icon(gtk.STOCK_DIALOG_INFO,
gtk.ICON_SIZE_LARGE_TOOLBAR))
notification.set_timeout(0)
# Show the notification
notification.show()

Running this program, you should see a notification if an appointment has an alert to be raised at that time.

This should be enough to start to build something.

If you don't want to program this into Python, you might want to take a look at gcalcli.